1. Field of the Invention
The invention relates to a metallurgical shaft furnace and in particular to the refractory construction of the bottom and the adjoining part of the hearth of a shaft furnace. The invention is especially applicable to blast furnaces.
2. Description of the Prior Art
U.S. Pat. No. 3,752,638 discloses a shaft furnace bottom having a graphite layer and, above the graphite layer, a layer of semi-graphite of coefficient of thermal conductivity (hereinafter referred to as the .lambda.-value) of about 20 kcal/m.h..degree. C. This semi-grahite layer is the uppermost layer of the bottom. Graphite has a high .lambda.-value of for example 90 kcal/m.h..degree. C. Below the graphite is a layer of carbon brick with .lambda.-value of about 4 kcal/m.h..degree. C. In an alternative construction proposed in this patent the semi-graphite layer is replaced by a carbon layer with .lambda.-value of 5 kcal/m.h..degree. C. covered by a magnesite layer having a .lambda.-value of 2 to 3 kcal/m.h..degree. C.
However, it has been found that when the shaft furnace is a blast furnace for the reduction of iron from iron ore, the carbon-free covering layer is affected by the high temperature drop across it, so that liquid pig iron comes into contact with the carbon layer. This carbon layer is gradually impregnated from top to bottom with iron, so its coefficient of thermal conductivity (.lambda.-value) tends to rise from about 4 to 5 to about 15 kcal/m.h..degree. C. As a result of this impregnation with liquid iron, and of the consequent increase in .lambda.-value the locations of the isotherms in it change. This leads to wear and attack on the carbon layer with the result that the liquid iron also reaches the graphite layer in places. The graphite layer which is highly expensive, is then also gradually affected.
For this reason, repairs and partial replacement of the bottom structure may be necessary at heavy expense, particularly on graphite bricks, and additionally the campaign life of the furnace is reduced, which leads to loss of production.